We propose to identify genes that underly susceptibility to asthma and atopy by performing two-point and complex linkage analysis with microsatellite markers in families with well-defined asthma. Our study will focus on 70 Chicago area families with two affected siblings with asthma. The diagnosis of asthma in all family members will be made using strict criteria based on medical history and results of spirometry and methacholine challenge studies; skin tests for inhaled antigens will be performed and total IgE, specific IgE, and eosinophil counts will be measured.